Semiconductor device

ABSTRACT

A semiconductor device includes a gate electrode GE electrically connected to a gate portion which is made of a polysilicon film provided in the inside of a plurality of grooves formed in a striped form along the direction of T of a chip region CA wherein the gate electrode GE is formed as a film at the same layer level as a source electrode SE electrically connected to a source region formed between adjacent stripe-shaped grooves and the gate electrode GE is constituted of a gate electrode portion G 1  formed along a periphery of the chip region CA and a gate finger portion G 2  arranged so that the chip region CA is divided into halves along the direction of X. The source electrode SE is constituted of an upper portion and a lower portion, both relative to the gate finger portion G 2 , and the gate electrode GE and the source electrode SE are connected to a lead frame via a bump electrode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 10/915,416 filedAug. 11, 2004 now U.S. Pat No. 6,930,354, which is a continuation ofapplication Ser. No. 10/458,668 filed Jun. 11, 2003 (now abandoned)

BACKGROUND OF THE INVENTION

This invention relates to a technique for semiconductor devices and moreparticularly, to a technique effective for application to semiconductordevices having power MISFET (metal insulator semiconductor field effecttransistor).

Transistors for high-power purposes dealing with electric power ofseveral watts (W) or over are called power transistor, and various typesof structures have been studied.

Among them, power MISFET includes ones called a longitudinal type and atransverse type and are classified into a trench-type structure and aplanar structure depending on the structure of a gate portion.

Such power MISFET has a multitude of (e.g. several tens of thousands of)MISFET's in fine pattern connected in parallel in order to obtain highpower.

For instance, Japanese Unexamined Patent Publication No. Hei7(1995)-249770 discloses power MISFET of the trench gate type.

SUMMARY OF THE INVENTION

We have engaged in studies and developments of power MISFET used as ahigh-efficiency power supply and the like.

With such power MISFET as mentioned above, it is required to reduce anON resistance (Ron), a gate capacitance (Qg) and, particularly, agate-drain capacitance (Qgd). A great current can be obtained byreducing the ON resistance. The reduction of the capacitance between thegate and drain leads to an improved switching switching characteristic.

Under these circumstances, studies have been made on the scale down ofpower MISFET and, particularly, on the reduction in width of a groovewhere a gate portion is formed.

More particularly, in order to reduce the ON resistance, it is necessaryto increase a channel area per unit area. If the width of the groove atwhich a gate portion is to be formed is made small, a channel area perunit area can be increased. If the width of the groove at which a gateportion is to be formed is made small, then a counter area for a drainportion at the side of a substrate opposite to the gate portion can bemade small, thereby ensuring reduction of the capacitance (Qgd).

However, when the width of the groove where the gate portion is to beformed, the resistance of the gate portion becomes large, therebycausing the switching characteristic to be degraded instead.

Especially, in high frequency operations, an efficiency η is greatlyinfluenced depending on the resistance of the gate portion as will behereinafter described in detail. This efficiency means a value of outputpower/input power.

Accordingly, a measure for reducing the resistance of the gate portionbecomes important.

An object of the invention is to provide a semiconductor device of thetype wherein the resistance of a gate portion of power MISFET isreduced. Another object of the invention is to provide a semiconductordevice of the type wherein a semiconductor device having power MISFETcan be improved in characteristics.

The above and other objects and novel features of the invention willbecome apparent from the following description with reference to theaccompanying drawings.

Typical embodiments of the invention among those embodiments set forthin this application are briefly described below.

A semiconductor device according to the invention comprises:

(a) a MISFET formed in a chip region of a semiconductor substrate andhaving a gate portion, a source portion and a drain portion, each madeof a first conductor;

(b) a gate electrode which is electrically connected with the gateportion and is made of a second conductor having a resistivity lowerthan the first conductor and which includes (b1) a first portion formedalong a periphery of the chip region and (b2) a second portion connectedwith the first portion and formed on the inside of said first portion insaid chip region;

(c) a source electrode which is electrically connected with the sourceportion and is made of the second conductor and which is formed plurallywithin the chip region;

(d) a bump electrode formed on the upper portions of the gate electrodeand the plurality of source electrodes, respectively;

(e) the gate electrode and the plurality of source electrodes beingarranged at the same layer level; and

(f) the second portion of the gate electrode being arranged betweenadjacent source electrodes of the plurality of source electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an essential part of a substrate showing afabrication method of a semiconductor device according to Embodiment 1of the invention;

FIG. 2 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 1 of the invention;

FIG. 3 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 1 of the invention;

FIG. 4 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 1 of the invention;

FIG. 5 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 1 of the invention;

FIG. 6 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 1 of the invention;

FIG. 7 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 1 of the invention;

FIG. 8 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 1 of the invention;

FIG. 9 is a plan view of the essential part of the substrate showing thefabrication method of a semiconductor device according to Embodiment 1of the invention;

FIG. 10 a sectional view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment1 of the invention;

FIG. 11 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment1 of the invention;

FIG. 12 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment1 of the invention;

FIG. 13 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment1 of the invention;

FIG. 14 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 1 of the invention;

FIG. 15 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment1 of the invention;

FIG. 16 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 1 of the invention;

FIG. 17 is a graph showing the relation between the gate resistance andthe efficiency for illustrating the effect of Embodiment 1 of theinvention;

FIG. 18 is a plan view of an essential part of a substrate showing afabrication method of a semiconductor device according to Embodiment 2of the invention;

FIG. 19 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment2 of the invention;

FIG. 20 is a plan view of an essential part of a substrate showing afabrication method of a semiconductor device according to Embodiment 3of the invention;

FIG. 21 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment3 of the invention;

FIG. 22 is a plan view of an essential part of a substrate showing afabrication method of a semiconductor device according to Embodiment 4of the invention;

FIG. 23 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 4 of the invention;

FIG. 24 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment4 of the invention;

FIG. 25 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 4 of the invention;

FIG. 26 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment4 of the invention;

FIG. 27 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 4 of the invention;

FIG. 28 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment4 of the invention;

FIG. 29 is a sectional view of the essential part of the substrateshowing the fabrication method of a semiconductor device according toEmbodiment 4 of the invention;

FIG. 30 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment4 of the invention;

FIG. 31 is a plan view of an essential part of a substrate showing afabrication method of a semiconductor device according to Embodiment 5of the invention;

FIG. 32 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment5 of the invention;

FIG. 33 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment5 of the invention;

FIG. 34 is a plan view of the essential part of the substrate showingthe fabrication method of a semiconductor device according to Embodiment5 of the invention;

FIG. 35 is a sectional view of the essential part of the substrate forillustrating the effect of Embodiment 1 of the invention;

FIG. 36 is a sectional view of the essential part of the substrate forillustrating the effect of Embodiment 5 of the invention;

FIG. 37 is a plan view showing an essential part of a substrate showinga further groove pattern according to an embodiment of the invention;and

FIG. 38 is a plan view showing an essential part of a substrate showinga still further groove pattern according to an embodiment of theinvention.

EMBODIMENTS OF THE INVENTION

The embodiments of the invention are described with reference to theaccompanying drawings, in which like members or parts having a similarfunction are indicated by like reference numerals throughout thedrawings illustrating the embodiments and are not repeatedly explained.

Embodiment 1

A semiconductor device according to this embodiment is described by wayof a fabrication method thereof.

FIGS. 1 to 16 are, respectively, a sectional or plan view of anessential part of a substrate showing a fabrication method of asemiconductor device according to the embodiment. The sectional viewcorresponds, for example, to a section taken along line A-A of a planview.

Initially, as shown in FIG. 1, a semiconductor substrate 1 (hereinafterreferred to simply as “substrate”), in which a single crystal siliconlayer 1 b doped with an n-type impurity (e.g. arsenic) is epitaxiallygrown on the surface 1 a of an n-type single crystal, is provided.

Next, as shown in FIG. 2, the surface of the substrate 1 is, forexample, thermally oxidized to form a silicon oxide film 3. Thereafter,a p-type impurity (e.g. boron) is injected over the silicon oxide film 3through a mask of a silicon nitride film (not shown) which has beenpatterned by use of a lithographic technique, followed by thermaldiffusion to form a p-type well 5. Next, the silicon nitride film isremoved.

Subsequently, as shown in FIGS. 3 and 4, the silicon oxide film 3 andthe substrate 1 are, respectively, etched through a mask of a filmpatterned by the use of a photolithographic technique to form grooves(trenches) 7. As shown in FIG. 4, the pattern of each groove 7 is in theform of a stripe extending along the direction of Y. CA indicates a chipregion. This chip region is in the form of a rectangle (oblong)elongated along the direction of X. It will be noted that although notshown in the figure, a great number of chips of the type as mentionedabove are provided on the wafer-shaped semiconductor substrate 1.

Next, as shown in FIGS. 5 and 6, the substrate 1 is thermally treated toform a thermally-oxidized film 9 on the bottom and side walls of eachgroove 7. This thermally oxidized film 9 serves as a gate insulatingfilm of power MISFET. Next, an impurity-doped, low resistancepolysilicon film 11 is deposited to such an extent that each groove 7 isburied. During the deposition, the polysilicon film 11 is formed as alayer on the silicon oxide film 3 over the p-type well 5. Thereafter,the polysilicon film 11 is etched through a mask of a photoresist film(hereinafter referred to simply as “resist film”) not shown, thepolysilicon film 11 is left inside the groove 7. The inside polysilicon11 acts as a gate portion G of power MISFET. Subsequently, a polysiliconfilm pattern P1 is formed over an outer periphery of a chip region CA,and a polysilicon film pattern P2 which is arranged to divide the chipregion CA into halves along the direction of X is also formed (FIG. 6).The patterns P1 and P2 are connected to each other. The region where thesilicon oxide film 3 is formed below the polysilicon film pattern P1serves as an element isolation region, and regions which are marked offwith this region are provided as an element forming region (active).

Next, the silicon oxide film 3 in the element forming region is removedand a thin silicon oxide film 13 is formed over the gate portion G andalso over each portion between the grooves 7 as is particularly shown inFIG. 7. Thereafter, a p-type impurity is injected into the substrate 1at the portion thereof between the grooves 7 and is diffused, therebyforming a p-type semiconductive region (channel region) 15. This p⁻-typesemiconductive region 15 extends to the inside of the p-type well 5.

Next, an n-type impurity (e.g. arsenic) is injected into the substrate 1at a portion between the grooves 7 through a mask of a resist film (notshown) and diffused to form an n⁺-type semiconductive region (sourceregion) 17. It will be noted that this n⁺-type semiconductive region(source region) 17 extends between the gate portions G, shown in FIG. 6,in the form of a stripe. Next, as shown in FIGS. 8 and 9, a siliconoxide film 19 is formed over the substrate 1, after which the siliconoxide films 13, 19 at a portion between the gate portions G and thesubstrate 1 (i.e. the p⁻-type semiconductive region 15 and the n⁺-typesemiconductive region 17) are, respectively, etched to form a contactgroove (source contact) 21 s.

This contact grove 21 s is so arranged as to permit the n⁺-typesemiconductive region 17 to be exposed from the side walls thereof andthe p⁻-type semiconductive region 15 to be exposed from the bottomthereof. In other words, the depth of the contact groove 21 s exceedsthe n+-type semiconductive region 17 and arrives at the p⁻-typesemiconductive region 15.

At this stage, the silicon oxide film 19 on the polysilicon filmpatterns P1 and P2 is removed to form contact grooves (gate contacts 21a, 21 b) (see FIG. 9). The contact groove on the pattern P1 is indicatedat 21 a and the contact groove on the pattern P2 is indicated at 21 b.

The contact groove 21 b is contacted at one end thereof (i.e. at a leftend as viewed in FIG. 9) with the contact groove 21 a, and the other end(i.e. a right end as viewed in FIG. 9) is not contacted with the contactgroove 21 a. More particularly, a space S1 is established between thecontact grooves 21 a and 21 b.

Next, as shown in FIGS. 10 and 11, a p-type impurity such as, forexample, boron fluoride (BF₂), is injected into the bottom of thecontact groove 21 s and diffused to form a p⁺-type semiconductive region(back gate contact region) 23. More particularly, the source electrodeformed on the contact groove 21 s is connected to the source region 17and further to the back gate via the p⁺-type semiconductive region 23.

In this way, the contact groove 21 s is formed and the p⁺-typesemiconductive region 23 is formed at the bottom thereof, so that anallowance for mask alignment can be reduced and spaces between the gateportions can be made finer over the case where a device having such astructure as shown in FIG. 35 is formed.

Next, according to a sputtering method, a TiW (titanium tungsten) film25 is thinly deposited, for example, as a barrier film over the siliconoxide film 19 including the insides of the contact holes (21 s, 21 a, 21b), followed by thermal treatment. Subsequently, an aluminium (Al) film27 is deposited, for example, as a conductive film according to aputtering method. This barrier film serves to prevent an undesirablereaction layer from being formed owing to the contact between Al and thesubstrate (Si). It will be noted that the Al film means a film mainlymade of Al, and other types of metals may be contained therein.

Next, the TiW film 25 and the Al film 27 are, respectively, etchedthrough a mask of a resist film not shown to form a gate electrode (gateleading electrode) GE and a source electrode (source leading electrode)SE. These electrodes (GE, DE) serve as a first-layer wiring.

As shown in FIG. 11, the gate electrode GE includes a gate electrodeportion (first portion) G1 formed along the periphery of the chip regionCA and a gate finger portion (second portion) G2 which is so arranged asto divide the chip region CA into halves along the direction of X. Thepattern of the gate electrode GE is shown in FIG. 12, and the pattern ofthe source electrode SE is shown in FIG. 13.

As shown in FIGS. 11 and 12, the gate electrode portion G1 is positionedon the polysilicon film pattern P1 and also on the contact groove 21 a.The gate finger portion G2 is positioned on the polysilicon film patternP2 and also on the contact groove 21 b.

It should be noted that any gate finger portion G2 is not formed on thespace between the contact grooves 21 a and 21 b (space S1).

On the other hand, the source electrode SE is constituted, as shown inFIGS. 11 and 13, of a portion which is located at one of the halves ofthe chip region CA divided with the polysilicon film pattern P2 (i.e. anupper side portion relative to the gate finger portion G2) and a portionlocated at the other half of the chip region (i.e. a lower side portionrelative to the gate finger portion G2). These portions are connected toeach other and combined together at the space S1. In other words, thehalves of the source electrode SE are connected in the vicinity of atthe end portion of the gate finger portion G2.

It will be noted that the gate electrode GE and the source electrode SEare at the same layer level, and a guard ring (not shown) may be formedat the outside of the gate electrode GE for protection of the element.

Next, as shown in FIGS. 14 and 15, a polyimide resin film 29 is appliedto over the substrate 1, for example, as a protective film, followed byexposure to light and development thereby removing the polyimide resinfilm 29 formed on the gate electrode GE and the source electrode SE toform openings (pad portions) 31 g, 31 s. The Al film 27 (the gateelectrode GE and the source electrode SE) is exposed at these openings.It is to be noted that although the opening 31 s does not appear at thesection taken along the line A-A of FIG. 15, the opening 31 s isillustrated in FIG. 14 in order to clarify the relation between thesource electrode SE and the opening 31 s.

Thereafter, the substrate 1 is protected such as with a tape on thesurface thereof, after which the substrate 1 is polished at the backside thereof so that the protecting face is turned downside as shown inFIG. 16. Subsequently, a Ni (nickel) film, a Ti (titanium) film and a Au(gold) film are successively formed, for example, as a conductive film,on the back side of the substrate 1 according to a sputtering method,thereby forming a built-up film 35 thereof. This built-up film 35 servesas a leading electrode (drain electrode DE) for drains (1 a, 1 b). Next,the tape is peeled off and a bump electrode made, for example, of goldor the like is formed at the openings 31 g, 31 s, respectively, followedby dicing the wafer-shaped substrate 1 along the chip region, packagingthe resulting individual chips on a lead frame (packaging plate) andsealing (packaging) with a resin or the like. Eventually, asemiconductor device is completed. With respect to bump-forming andpackaging procedures, these are described in detail in Embodiment 4 orthe like and are not illustrated herein.

Thus, according to this embodiment, the gate finger portion G2 isdisposed in the gate electrode GE, so that the gate resistance Rg can bereduced. As a result, switching characteristics can be improved.

Especially, because the gate-drain capacitance (Qgd) is reduced, thegate resistance can be reduced if the width of a groove where the gateportion is formed is made small (particularly, in the case where thegroove is formed as a stripe). Thus, it becomes possible to reduce thegate-drain capacitance (Qgd), ensure high-speed switching operation dueto the reduction of the gate resistance and reduce a switching loss.

Especially, where the groove pattern is formed as a stripe, it ispossible to reduce the gate resistance according to this embodiment withthe tendency that the gate resistance Rg increases in proportion to thepattern length of the groove over the case using a pattern of FIG. 37described hereinafter because the cells arranged in parallel becomesmall in number.

Upon driving LSI, there is a tendency toward low voltage and greatelectric current. For instance, with CPU of a notebook computer, studieshave been made to design a drive voltage at about 1.6V and an appliedcurrent at about 20A. In addition, miniaturization of a notebookcomputer and the like has been greatly required, for which a workingfrequency (f) is in a high frequency range of from 300 kHz to 500 kHz.Studies on the breakdown of a loss of a synchronous rectifying circuitused as a power supply for notebook computers and constituted of powerMISFET reveal that the sum of “on” loss and switching loss is at 50% orover. Accordingly, it will be seen that the reduction of these lossescontributes greatly to high efficiency.

FIG. 17 is a graph showing the relation between the gate resistance Rg(Ω) and the efficiency η (%). As shown in the figure, the efficiency ata frequency of 1 MHz becomes lower than that at 300 kHz. In either case,the efficiency increases with a decreasing gate resistance. However, itwill be seen that with the case where the frequency f is at 1 MHz, thegradient in the graph is sharper than that at 300 kHz, and an increasingrate of the efficiency based on the lowering of the gate resistance isgreat. It will be noted that the input potential Vin is at 12V, the outpotential Vout is at 1.6 V and the output current Iout is at 10A.

Accordingly, in power MISFET corresponding to a high frequencypotential, it is convenient to use such a structure as stated in thisembodiment.

It will be noted that extensive studies on the structure have been madewherein a gate finger portion G2 and the like are disposed, revealingthat the gate resistance can be suppressed to 1 Ω or below.

Upon comparison with an existing structure checked by the inventors(i.e. a structure having such a pattern of a groove 7 as shown in FIG.38 and not formed with the gate finger portion G2), the device havingthe structure according to this embodiment exhibits an improvedefficiency of about 2% at 300 kHz and from 2 to 4% at 1 MHz.

In this manner, the gate resistance Rg can be reduced and thus, theefficiency can be improved.

It will be noted that the shape in pattern of the contact groove may beany one which allows the connection between the gate electrode GE andthe gate portion G, and also between the source electrode SE and then⁺-type semiconductive region 17 and should not be construed as limitingto the shape which has been illustrated with reference to FIG. 9.However, in order to reduce the gate resistance Rg and the like, it isas a matter of course that the contact area of these portions shouldpreferably be greater.

Although the polysilicon film is used as a gate portion in thisembodiment, other types of films including a silicide film and acomposite film of polysilicon and silicide may be likewise used.

Embodiment 2

Although the space S1 is provided between the contact groove 21 a and 21b (see FIG. 9) and any gate finger portion G2 is not formed on the spaceS1 (see FIGS. 11, 12) in Embodiment 1, the contact grooves 21 a and 21 bmay be connected to each other and the gate finger portion G2 and thegate electrode G1 may be connected together over the space S1.

FIG. 18 shows a pattern of contact grooves (21 a, 21 b, 21 s). FIG. 19shows a pattern of a gate electrode GE and a source electrode SE. Asshown in FIG. 19, the source electrode SE is divided into halves.

It will be noted that because those other than the patterns of thecontact grooves and the gate electrode GE and the source electrode SEare similar to in Embodiment 1, the structures of the respective membersand procedures for forming same are not described herein.

Embodiment 3

The case where only one gate finger portion G2 is formed along thedirection of X (see FIGS. 11, 12) has been illustrated in Embodiment 1.Alternatively, an increasing number of gate fingers may be provided asdescribed below.

FIG. 20 shows a pattern of a gate electrode GE and a source electrode SEwherein two gate finger portions G2 are provided.

FIG. 21 is a pattern of a gate electrode GE and a source electrode SEwherein three gate finger portions G2 are provided.

It will be noted that contact grooves (21 a, 21 b) may be formed in thesame pattern as the gate electrode GE.

In this way, an increase in number of the gate finger portions allowsthe gate resistance Rg to be efficiently reduced. Especially, it ispreferred that where the gate portion G is elongated along the directionof Y correspondingly to the size of the chip region, the number of gatefingers is increased to reduce the gate resistance.

In FIGS. 20 and 21, any gate finger G2 is not provided over the spaceS1. Alternatively, as illustrated with respect to Embodiment 2, the gatefinger portion G2 may be connected to the gate electrode portion G1 overthe space S1.

Embodiment 4

In this embodiment, the procedure of forming bump electrodes on theopenings 31 g, 31 s illustrated in Embodiment 1 and mounting a chip isdescribed.

FIGS. 22 to 28 are, respectively, a sectional or plan view of anessential part such as of a substrate showing a method of fabricating asemiconductor device according to this embodiment.

Initially, a substrate 1 of the type, which has been described inEmbodiment 1 with reference to FIGS. 14 and 15, is provided. As shown inFIGS. 22 to 24, bump electrodes 37 g, 37 s made of a metal such as goldor the like are formed on the openings 31 g, 31 s, respectively. FIG. 23is a sectional view schematically showing the state of the substrate inthe vicinity of the opening 31 s, and FIG. 24 is a plan view of anessential part of the substrate. FIG. 23 corresponds to the sectiontaken along line B-B of FIG. 24.

Reference numeral 37 g indicates a bump electrode for connection with agate electrode DE, and reference numeral 37 s indicates a bump electrodefor connection with a source electrode SE. These bump electrodes 37 s,37 g can be formed, for example, by placing balls of molten gold overthe openings 31 g, 31 s, respectively.

Next, the wafer-shaped substrate 1 is diced, for example, in arectangular form along the chip region.

Thereafter, as shown in FIGS. 25 and 26, a chip CH is bonded to andfixed on a lead frame R1 at the back side thereof by use of a silver(Ag) paste 39 or the like. The lead frame R1 has a chip mounting portionR1 a and an external terminal R1 b. In doing so, the lead frame R1 andthe back side (drain electrode DE) of the chip CH are electricallyconnected to each other. More particularly, the external terminal R1 bbecomes a drain terminal DT.

On the other hand, the chip CH has a lead frame R2 mounted on thesurface side thereof, followed by thermocompression to permit the bumpelectrodes 37 s, 37 g and the lead frame R2 to be bonded together. Thelead frame R2 has four external terminals R2 a to R2 d, of which theexternal terminals R2 b to R2 d are electrically connected to the bump37 s, respectively, and the external terminal R2 a is electricallyconnected to the bump electrode 37 g. More particularly, the externalterminal Ra serves as a gate terminal GT and the external terminals R2 bto R2 d serve as a source terminal ST.

Subsequently, as shown in FIGS. 27 and 28, a resin melt 41 is chargedand cured, for sealing, between the chip CH and lead frame R2 and overthe lead frame R2.

According to this embodiment, because the connection with the externalterminals R2 a to R2 d is established by use of the bump electrodes, theconnection resistance between the source electrode SE or gate electrodeGE and each of the external terminals R2 a to R2 d can be reduced.

For instance, although these connections are possible by use of a wiresuch as of a gold wire, the resistance of the gold wire and theinductance of a source or a gate undesirably increase.

If this inductance is great, (1) a transient induced voltage occurs.This voltage acts as a negative feedback relative to gate drive voltageand increases an ON resistance in the course of the period oftransition. Moreover, 2) an impedance between source and drainincreases, adversely influencing the transient characteristics underworking conditions at a large electric current and at a high value ofdi/dt. Such problems as mentioned above are more frequently experiencedas the frequency becomes higher.

In contrast, according to this embodiment, the inductance can bereduced, with an improved efficiency and improved devicecharacteristics. It will be noted that as a result of the inventors'study, the device of this embodiment has an efficiency higher by about 1to 2% than a package using a gold wire.

Especially, as described in detail in Embodiment 1, if the resistance Rgof the gate portion is reduced by trying various measures for thearrangement of the gate electrode and the source electrode, thereduction effect of the gate resistance Rg cannot be satisfactorilyshown under high frequency working conditions in case where theresistance or inductance becomes large in association with externalterminals.

Accordingly, the package form (or package structure) set forth in thisembodiment may be one which is suitable for use in power MISFET set outin Embodiment 1 and the like. Of course, this package structure may beapplicable to structures different from the power MISFET described inEmbodiment 1.

Embodiment 5

In Embodiment 4, although the bump electrodes have been formed on thegate electrode GE and the like, a bump electrode may be formed afterformation of a conductive film made, for example, of Al on the gateelectrode GE as described below.

FIGS. 29 and 30 is, respectively, a sectional view of an essential partof a substrate and the like showing a semiconductor device according tothis embodiment.

First, a substrate 1 of the type, which has been illustrated withreference to FIGS. 14 and 15 in Embodiment 1, is provided. As shown inFIGS. 29 and 30, a conductive film such as, for example, an Al film (33)is deposited over a polyimide resin film 29 including the insides of theopenings 31 g, 31 s.

Next, the Al film (33) is so patterned as to be larger in size than theopenings 31 g, 31 s. Reference numeral 33 g indicates an Al film overthe opening 31 g and reference numeral 33 s indicates an Al film overthe opening 31 s.

Thereafter, bump electrodes 37 g, 37 s are, respectively, formed on theAl films 33 g, 33 s in the same manner as in Embodiment 4.

The wafer-shaped substrate 1 is diced and individual chips are packaged.These steps may be performed in the same manner as in Embodiment 4 andare not described again.

As will be apparent from the above, the Al films 33 g, 33 s are,respectively, formed below the bump electrodes 37 g, 37 s in addition tothe Al film 27 constituting the gate electrode and the source electrodein accordance with this embodiment, so that stress exerted upon theformation of the bump electrodes 37 g, 37 s or upon the connection ofthe lead frames R1, R2 can be mitigated, thereby reducing a bondingdamage.

With packaging using a so-called wire bonding technique, stress isexerted on chips only when wire bonding is performed. Accordingly, theneed for applying the embodiment of the invention to the technique isnot great. Nevertheless, the technique set out in this embodiment isimportant for the package using bump electrodes because stress isapplied to not only at the time of formation of bump electrodes, butalso at the time of connection (thermocompression) between the leadframe and the bump electrode, under which bonding damage is liable tooccur.

This stress may be mitigated by forming a thick Al film (27)constituting the gate electrode and the source electrode. However, ifthe Al film is formed as being thick, a subsequent processing step (i.e.for the formation of gate and source electrodes) becomes difficult.

Especially, as illustrated in Embodiment 1 or the like, where the gatefinger portion G2 is provided at the gate electrode GE, the patternbecomes complicated in shape, so that the procedure of this embodimentwherein the Al films are built up is appropriate.

The Al films 33 g, 33 s are formed only over the openings 33 g, 33 s,under which film stress exerted on the substrate can be mitigated. Moreparticularly, when the forming areas of the Al films 33 g, 33 s areincreased, film stress becomes great, causing the substrate to bestrained or cracked.

In the power MISFET illustrated in Embodiment 1 or the like, a bumpelectrode is formed over the gate portion G and the gate portion G has atrench structure. In this case, stress is liable to be exerted on theupper end portion of the groove 7 shown in FIG. 22, thereby causing gatebreakage. In the case, it is favorable to use conductive films (i.e. theAl films 33 g, 33 s) of this embodiment for stress relaxation. It willbe noted that the technique of this embodiment is applicable to thosestructures different from power MISFET set forth in Embodiment 1.

Embodiment 6

Many tests (inspections) are usually conducted in the course offabrication of semiconductor devices or after completion of the devices.

For instance, whether or not power MISFET works properly is inspected byapplying a given potential to the gate electrode GE or source electrodeSE that has been illustrated with reference to FIG. 11 in Embodiment 1.This inspection is called a probe inspection, which is carried out byapplying a potential via a probe to electrodes of individual chipregions of a wafer-shaped substrate.

This probe inspection may be carried out, for example, by bringing aprobe into contact with the gate electrode GE and the source electrodeSE exposed from the openings 31 g, 31 s shown in FIG. 15. In this case,however, a probe trace (a scar with the probe) is left on the surface ofthe gate electrode and the like.

If bump electrodes (37 g, 37 s) are formed on the probe traces,connection failure or the lowering of connection strength may be caused.

To avoid this, according to this embodiment, an opening 31 p for probeinspection (or for measurement) is provided aside from the openings forbump electrodes (FIG. 31). A plurality of openings 31 p may be provideddepending on the number of inspections. For example, a sense inspectionterminal, a focus inspection terminal and the like may be provided.

In the MISFET of this embodiment, a pad for probe inspection is formedon a so-called active region in order to make effective use of the chiparea. The term “on active region” means, for example, a region that ismarked out with the silicon oxide film 3 formed beneath the gateelectrode GE shown in FIG. 16. The active region is partitioned withthis silicon oxide film 3 substantially in a rectangular form. In otherwords, the active region is surrounded with the silicon oxide film 3.

In contrast, a pad for probe inspection may be provided, for example, ata region other than the active region (e.g. a stripe region extending toaround the active region or between the chip regions). However, such aregion is very narrow, making it difficult to provide the pad for probeinspection. If a pad for probe inspection is provided in a peripheralregion, the chip size becomes great, so that the number of chipsobtained form one wafer is reduced. This eventually leads to highermanufacture costs.

FIG. 31 is a plan view of an essential part of the substrate after theformation of openings. An opening 31 p is formed over the sourceelectrode SE simultaneously with the formation of the openings 31 s, 31g for bump electrode shown in the figure. It will be noted that all thesteps before the step of forming the polyimide film 29, inclusive, arecarried out in the same manner as in Embodiment 1 and are not describedagain. After the formation of the opening 31 p, such an opening is usedfor probe inspection, after which bump electrodes are formed in the samemanner as in Embodiment 4, followed by dicing and packaging. Of course,an opening for probe inspection may be formed over the gate electrode.

The pattern of the opening 31 p is in a rectangular (or oblong) form.The rectangle-shaped pattern permits easy contact of a probe and areduced opening area. It should be noted that any bump electrode 31 p isnot formed over the opening 31 p, which is covered, for example, with aresin 41.

As shown in FIG. 36, for example, in case where any gate finger portionG2 is not formed, a large-sized opening 50 s can be formed within a chipregion CA. In this case, it is unlikely that a bonding portion is formedon a probe trace.

Accordingly, as illustrated in the above embodiment, where the gatefinger portion G2 is provided or where connection with an externalterminal through a bump electrode is intended, it is preferred to forman opening 31 p for probe inspection.

As having illustrated in Embodiment 2, where the gate finger portion G2and the gate electrode portion G1 formed along the periphery of the chipregion CA are connected with each other over the space S1, it isfavorable to form an opening 31 p for probe inspection on individualhalves obtained by division of the source electrode SE.

In this way, when the opening 31 p for probe inspection is formed oneach of the divided source electrode SE, element characteristics of therespective region can be accurately obtained.

On the contrary, where the patterns of the source electrode SE areintegrally connected with each other, it is convenient to form anopening 31 p for probe inspection in any of the regions and the probeinspection time (number of inspections) can be reduced.

In this embodiment, although the case where the electrode is dividedinto halves (FIG. 31) has been illustrated, this embodiment isapplicable to the case where the electrode is divided into plural piecesas illustrated in Embodiment 3. FIG. 33 shows an instance of a layout ofopenings 31 p for probe inspection in case where two gate fingers G2 areprovided. Likewise, FIG. 34 shows an instance of a layout of openings 31p for probe inspection in case where three gate fingers G2 are provided.

According to this embodiment, unlike the case shown in FIG. 36, thesource electrode SE and the like are widely covered with a polyimideresin, so that a satisfactory contact area with a sealing resin isensured, thereby preventing moisture or the like from entering into theopenings.

It will be noted that this embodiment is applicable to a structure otherthan that of power MISFET set forth in Embodiment 1.

Although the invention made by us has been particularly described basedon the embodiments, the invention should not be construed as limitingtop these embodiments and many changes are possible without departingfrom the spirit of the invention.

Especially, in the foregoing embodiments, the case where the groove 7 isin a striped pattern has been set out. Besides, the groove may be formedin such patterns as shown in FIGS. 37 and 38, respectively. FIG. 37shows a case where the external shape of the groove 7 is inoctagon-shaped mesh, and FIG. 38 shows a case where the external shapeof the groove 7 is in square-shaped mesh.

The effects of typical embodiments disclosed in the present inventionare briefly described below.

A gate finger (second portion) is provided at a gate electrode of powerMISFET, so that the gate resistance can be reduced, thus leading toimproved characteristics of semiconductor device.

The gate electrode and source electrode of power MISFET are connected toexternal terminals by use of bump electrodes, so that thecharacteristics of semiconductor device can be improved.

1. A semiconductor device comprising: a MISFET formed in a chip regionof a semiconductor substrate, said MISFET having a gate portioncomprised of a first conductor formed in a groove formed in a firstsurface of the semiconductor substrate, a source portion formed at saidfirst surface, and a drain portion formed at a second surface oppositeto said first surface; a gate electrode and a source electrodeelectrically connected to said gate portion and said source portion,respectively; a protective film having first and second openings formedover said gate electrode and said source electrode, respectively; bumpelectrodes formed at the first and second openings over said gateelectrode and said source electrode; and a third opening which exposesthe source electrode, formed in said protective film; wherein said gateelectrode and source electrode are formed in a same layer; said bumpelectrodes are electrically connected to said gate electrode and saidsource electrode, respectively; said third opening is used for aninspection process; said groove has a stripe form extending along afirst direction; said gate electrode has a first portion and a secondportion; said first portion of said gate electrode is formed along aperiphery of said chip region; said second portion of said gateelectrode is formed in an area which is surrounded by said first portionof said gate electrode; and said second portion of said gate electrodeextends in a second direction which intersects the first direction. 2.The semiconductor device according to claim 1, wherein said first andsecond openings have substantially a circular form in plan.
 3. Thesemiconductor device according to claim 1, wherein said third openinghas substantially a rectangular form in plan.